Medical coma


The patient in coma who is brought to the hospital casualty department, or seen on the intensive care unit, though not having been exposed to evident trauma, may be harbouring delayed effects of head injury such as a subdural haematoma or meningitis arising from a basal skull fracture. The possibilities of raised intracranial pressure following a parenchymal haematoma in a hypertensive patient, the decompensation of a cerebral tumour, or the collection of pus means that all possible causes of loss of consciousness must be considered in coma. In the diagnosis of medical coma it is not easy to exclude the possibility of head injury.

If patients with a transient loss of consciousness following seizure, syncope, cardiac dysrhythmia or hypoglycaemia, and those unresponsive due to impending death are excluded, then, once a patient has been unconscious for 5-6 hours, about 40% of such patients seen in medical practice will have taken some form of sedative drug with or without alcohol.1 Of the remainder just over 40% will have suffered a hypoxic-ischaemic insult as a result of cardiac arrest or anaesthetic accident, a third will be unconscious as a result of cerebrovascular accident, either haemorrhage or infarction, and about a quarter as a result of metabolic coma, including infection, renal or hepatic failure, and complications of diabetes mellitus. If one considers only those cases which are initially regarded as of "unknown aetiology" the proportion of drug overdoses is about 30%, mass lesions 35%, and diffuse metabolic causes 35%.2

Few problems are more difficult to manage than the unconscious patient because the potential causes of loss of consciousness are considerable and because the time for diagnosis and effective intervention is relatively short. All alterations in arousal should be regarded as acute and potentially life threatening emergencies until vital functions are stabilised, the underlying cause of the coma is diagnosed, and reversible causes are corrected. Delay in instituting treatment for a patient with raised intracranial pressure may have obvious consequences in terms of pressure coning but, similarly, the unnecessary investigation by imaging techniques of patients in metabolic coma may delay the initiation of appropriate therapy. It is therefore essential for the physician in charge to adopt a systematic approach, initially to ensure resuscitation then to direct further tests towards producing the most rapid diagnosis and the most appropriate therapy. The development of such a systematic approach demands an understanding of the pathophysiology of consciousness and the ways in which it may be deranged.

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